Innovations in the field of laser xerography have always been motivated by the desires for more detailed and accurate printing. The high resolution of laser printers is one of their most beneficial features. However, the resolution of these printers has limitations. While 300 dot per inch printers are becoming commonplace, increasing the resolution further requires larger and more complex systems.
Most laser printers use an Organic Photoconductor (OPC) drum which has a photoconductive coating upon which an electrostatic charge is stored. Once a charge is placed on the OPC drum, a laser source is scanned across the surface of the drum to discharge specific regions of photoconductor. Depending on whether the printer is a "write white" or a "write black" type printer, the resulting charged and discharged regions of the drum define what will be dark or light regions of the resulting image. Thus, the image is in essence scanned onto the surface of the drum with the beam of the laser.
The terms "write black" and "write white" when describing laser printers refer, respectively, to whether the toner of the printer is to stick to the charged regions of the drum or the regions discharged by the laser. For example, using a toner with a charge opposite that of the drum results in the toner sticking to the drum in the charged regions and being repelled from the discharged regions. However, if a toner having the same charge as the charged regions of the drum is used, the toner is repelled from the charged regions. This toner remains in the discharged regions due to friction and the build up of charges in the discharged regions which have the opposite polarity to the toner.
Once toner is on the OPC drum in the desired locations, a sheet of paper is then passed by the drum, and by providing a strong electrical charge behind the paper, the toner is transferred from the OPC drum to the paper. The distribution of the toner on the page is a print of the distribution on the drum, creating the desired printed image on the page.
The use of the laser to scan the desired image onto the OPC drum allows a high resolution image to be formed. However, this resolution is typically limited by the resolution of the image data from the data output source. Nearly all standard software, for both text and graphics, uses a "bitonal" data output scheme. In a bitonal system, each image pixel is represented by a single data element (i.e. a bit), and is therefore either "on" or "off". Thus, the printer when scanning the image to be printed onto the OPC drum, is limited by the pixel resolution of the software, and each printed pixel is either completely darkened or completely blank.
A known method of increasing the quality of a printer output without increasing the resolution is through the use of a technique called anti-aliasing. In anti-aliasing, the high frequency components are filtered out of the printout and smoother edges and curves result. Different pixel locations on a printed page can be given different levels of darkness (or grayscale) by controlling the size of the printed spot in the pixel location. Thus, a grayscale capable printer can print a much smoother and clearer image by varying the tone of the printing in selected regions.
To adequately print a grascale image requires accurate control of the laser source of the printer to ensure the scanning of the proper area within a pixel region. Besides the control problems particular to the anti-aliasing grayscale printing method, the laser diode is known to be a sensitive device, and is easily destroyed by voltage spikes and overheating. In addition, its transfer curve changes radically depending on the junction temperature of the device. Thus, a properly controllable modulation circuit is imperative to ensure the desired scanning of the pixel regions of the grayscale printer, and to maximize the performance of a laser diode of the printer.